PROJECT SUMMARY The goal of this project is to understand the contribution of persistent immune signaling to brain injury and repair in former National Football League (NFL) players through imaging and the study of circulating cytokines. We are concerned that such individuals develop cognitive impairment at a higher rate than the general population, which may be generalizable to those participating in other sports or with other forms of repeated traumatic brain injury (TBI). We focus on measuring the activity of microglia, the resident immune cells of the CNS, because of their importance in responding to brain injury. Based on published evidence and our preliminary data, we hypothesize that former NFL players have functionally hyper-activated microglia located in brain regions vulnerable to injury from collision sports. Such injury is marked by increased expression of translocator protein 18 KDa (TSPO) by microglial cells and reactive astrocytes. We further hypothesize that prolonged microglial activation in regions of repeated axonal injury causes neuronal energy and functional deficits that are mechanistically linked to neurodegeneration. We recently showed that [11C]DPA-713 (DPA) positron emission tomography (PET) can be used to measure increased expression of TSPO, a marker of brain injury and repair, in human neurodegenerative disease. In the first study of TSPO in NFL players, we found higher DPA binding in the brains of elderly players compared to elderly controls. Our newer published findings also reveal higher DPA binding in a cohort of young, active or recently retired NFL players compared to a control group of non-collision sport athletes in several of the same cortical and mesial temporal lobe structures tested in the published pilot of older players. Two young players recently returned for two-year follow-up imaging that revealed stable TSPO distribution in all brain regions tested, and one of them also showed PET-based evidence of increased tau burden in several brain regions at this second visit. He was among eight of 15 young NFL players with high peripheral pro-inflammatory marker profile at his baseline DPA imaging, supporting the hypothesized link between pro-inflammatory signaling and vulnerability to aberrant tau deposition after repeated TBI. We now propose to measure the distribution of TSPO using DPA PET in the brains of 35 recently former NFL players compared to a control group of 35 healthy, non-collision sport athletes (Aim 1) in parallel with biofluid (CSF, plasma) assays for markers of inflammation in the same population (Aim 2). The (two-year) persistence of these immune markers will be tested in Aim 3. Our design uses DPA, which has advantages over other 2nd-generation radiotracers for imaging TSPO. Our infrastructure for research of carefully selected young, former NFL players and controls is unique and yet aligns with methodology of other groups studying elderly NFL players. By characterizing the persistent inflammatory response in the brains of young, former NFL players, we will provide a basis for understanding ensuing symptomatology, informing prognosis, and suggesting new therapies that may generalize to other populations with TBI.